Why Doesn’t Stainless Steel Rust?

Steel has been an invaluable industrial material for millennia. The earliest known use of steel dates back nearly 4,000 years to to an archaeological site in Anatolia. The Roman military were known for thier use of steel, and other early civilizations used it for tools, weapons and general construction. Steel was particularly valuable to these early civilizations, both for its strength and relatively lightweight properties, however it possessed one key flaw that was a tremendous drawback: it was incredibly susceptible to rust.

Fortunately, stainless steel was developed more than a century ago and due to it’s amazing corrosion resistance, it has welcomed a new era of industry and design. Manufacturers are now able to create all kinds of products and structures without the worry that the metal will succumb to rust when exposed to moisture. There are even certain stainless steel alloys that are suitable for marine and other extreme environments.

So, what gives stainless steel its protection from rust?

Iron Is Prone To Corrosion

A common mistake is associating any form of corrosion with rust, when the truth is that rust only occurs with iron and iron bearing materials. The reason is that rust is, by definition, iron oxide. This red oxide is the result of a chemical reaction taking place specifically between iron, oxygen, and water. And it doesn’t take much, even just the moisture in the air can be enough to corrode iron, as it is naturally susceptible to the reaction.

Given enough time in the presence of oxygen and water, iron will eventually be converted entirely into rust and deteriorate completely. This is due to the fact that the iron oxide or ‘rusting ‘surface creates flakes and offers little to no protection to the underlying iron. Pure iron will corrode rather quickly, but it is important to note that almost all metals that contain iron will eventually succumb to rust; this includes carbon steel, which is an alloy consisting mainly of carbon and iron.

The chemical process of rusting starts with the penetration of the iron’s surface by water molecules. It may seem like the metal is solid, but its microscopic cracks leave it exposed. This allows the hydrogen atoms in the water to combine with molecules in the metal. As the process proceeds, more and more of the metal becomes exposed. If chloride is present in the water, as with salt water, the corrosion occurs much more quickly.

As this is happening, the oxygen atoms bind with the iron atoms to form the destructive iron oxide compound. This causes the metal to weaken and become flaky and brittle.  Its typical red color has become known as rust.

Stainless Steel Was Developed as a “Rustless Steel” Solution

Stainless steel is unique to other steels in that it is characterised by the presence of chromium as an alloying agent. A typical batch of stainless steel will contain a minimum of 10.5 percent chromium versus only 1.5 percent carbon. The presence of the chromium and other alloying agents effectively prevents the rusting that normally occurs with carbon steel.

Scientists first became aware of the corrosion resistant properties of iron-chromium alloys as far back as the 1820’s.  For many years metallurgists of the time were not able to find the right proportion of carbon and chromium, and the early alloys were far too brittle to be sold on the market. It wasn’t until the 1870’s that John T. Woods and John Clark discovered the first modern example of stainless steel, which was patented in England.

Over the next 42 years, researchers continued to experiment and develop the different versions of early stainless steel. It was only in 1912 that Harry Brearley developed the first martensitic stainless steel alloy that was commercially viable. His own company, called Firth Vickers, eventually sold the metal under the Staybrite brand in England and it was used for the new entrance canopy of the Savoy Hotel in London. An American patent was finally granted in 1915, and the official era of stainless steel had firmly begun.

How Does Stainless Steel Prevent Corrosion?

It is normally assumed that stainless steel is impervious to its immediate environment in a way that mild steel is not, but the truth is stainless steels alloying agents are actively interacting with the environment immediately around the metal. It is this very reaction that protects stainless steel from rusting. The important alloying agents in the stainless steel, chromium and molybdenum, combine with the oxygen in the water and/or air to form a thin, stable film of metal oxide on the metals surface.

This film forms a protective layer on the steel and protects it from visible and damaging corrosion such as rust. This film is thin, so thin that wavelengths of light can’t even interact with it, and it is typically only visible with the aid of modern magnification instruments. This results in the metal retaining its aesthetically pleasing appearance and giving the illusion of being “stainless.”

What happens if this layer get damaged? Well when the surface of the steel is scratched or damaged, the exposed inner layer quickly reacts with the air or water and forms a new protective layer. This is due to the protective layer not being an addition to the steel but rather a property of the metal itself. In this way stainless steel is essentially ‘self-repairing’.